Data collected in the CPP are being analysed to determine primary and contributing roles of biological and environmental factors in mental retardation in a population of 40,000 children followed from the prenatal period to age 7. The incidence of severe retardation (0.5%) did not differ by ethnic group, but mild retardation as more frequent among blacks and Puerto Ricans (5%) than among whites (1%). Approximately 70% of the severely retarded, but less than 25% of the mildly retarded children had major neurological problems. Perinatal anoxia was associated with mental retardation at all ages, and psychomotor test scores in infancy were good predictkrs of school-age retardation. Mild retardation was associated with lower socioeconomic status. teristic